The present invention relates to a method for power and/or voltage control in a synchronous machine, and a synchronous machine for power and/or voltage control.
In the following xe2x80x9csynchronous machinexe2x80x9d shall be taken to mean synchronous generator. Synchronous generators are used in electric power networks in the first place to supply active and reactive power in the xe2x80x9chour scalexe2x80x9d. Active power can also be controlled in the xe2x80x9csecond-minute scalexe2x80x9d (frequency control), as well as reactive power (voltage control). Synchronous machines also provide suitable contributions in the xe2x80x9cmillisecond scalexe2x80x9d to the fault currents, so that error states in the network can be quickly resolved in selective manner.
Synchronous machines are important production sources of reactive power in power systems. When the reactive power requirement increases in the system, this tends to lower the terminal voltage on the synchronous machine. To keep the voltage constant, the field current is normally increased by means of the voltage regulator of the synchronous machine. The synchronous machine will thus produce the reactive power required to achieve reactive power balance at the desired terminal voltage.
The above mentioned process applies as long as the power production corresponds to one point in the permissible area in the capability graph of the synchronous machine, i.e. the graph of limits as regards reactive and active power, see FIG. 1 showing the relationship at overexcited operation. At overexcited operation, i.e. when the synchronous machine is producing reactive power, the permissible operating area is limited by thermally based rotor and stator current limits. The synchronous machines of today are normally dimensioned so that rotor and stator current limits intersect each other at a point at rated power factor A, see FIG. 1. The rated power factor for synchronous generators is typically 0.8-0.95. At overexcited operation, where the power factor is greater than the rated power factor, the limit for the capability graph of the synchronous machine consists of the stator current limit and, at overexcited operation, where the power factor is less than the rated power factor, the limit consists of the rotor current limit.
In conventional technology, if the stator or rotor current limits are exceeded current limiters, if such are installed and used, come into operation. These limiters reduce the currents by lowering the excitation. Since it takes a certain time before damaging temperatures are obtained, intervention of the current limiters of the stator or rotor is delayed several seconds before the current is lowered. The delay typically depends on the size of the current but it is usually less than one minute, see e.g. VERIFICATION OF LIMITER PERFORMANCE IN MODERN EXCITATION CONTROL SYSTEMS in IEEE Transaction on Energy Conversion, Vol. 10, No. 3, September 1995. The current reduction is achieved by a decrease in the field current which results in a decrease in the terminal voltage and reactive power production of the generator. The consequences for the part of the system in the vicinity of the machine are that the local reactive power production decreases and that it is more difficult to import power from adjacent parts of the system, when the voltage drops.
If the transmission network is unable to transmit the power required at prevailing voltages there is a risk of the power system being subjected to voltage collapse. To avoid this it is advantageous for the power to be produced locally, close to the load. If this is not possible, and the power must be transmitted from other parts of the system, it is, as known, advantageous if this can be done at as high a voltage level as possible. When the voltage drops, the reactive power production (shunt capacitances) of the transmission lines decrease. Transformer tap-changers act in order to keep the voltages to the loads constant, and thus the power of the loads constant. If the power consumption of the loads is constant and the transmission voltage is lower than normally, the currents in the transmission lines will be higher and the reactive power consumption of the transmission lines will be greater (series inductances), see Cigrxc3xa9 brochure 101, October 1995.
In many power systems, if current limiters come into operation for certain synchronous machines as described above, the reactive power production is limited and this may lead to a voltage collapse of the system.
In normal operation of the power system, with an essentially intact network, these situations are normally avoided by the installation of additional reactive power production resources, e.g. mechanically switched shunt capacitors and/or thyristor controlled static var compenstors (SVC), if necessary. However, as a widespread voltage collapse usually has severe consequences for the society, also abnormal operating conditions needs to be considered. If the network is weakened, due to e.g. faults or maintenance on important elements of the network, the installed reactive power producing resources may no longer be sufficient, resulting in the above described situation which may lead to voltage collapse. The cost of installing additional controllable reactive power producing resources, e.g. SVC devices, such that also these abnormal operating conditions can be handled is considerable. There is consequently a need for inexpensive controllable reactive power production reserves. These reserve resources should be capable of delivering reactive power such that voltage can be maintained at prescribed levels for at least 10 to 20 minutes giving the system operators a chance to take preventive actions, such as e.g. starting gas turbines or shedding load.
In power systems known today, or in power plants, the energy conversion usually occurs in two stages, using a step-up transformer. The rotating synchronous machine and the transformer, each have a magnetic circuit. It is known that manufacturers of such equipment are cautious and conservative in their recommendations for the set values in the limit devices, see Cigrxc3xa9 brochure 101, October 1995, section 4.5.4., page 60. Coordination is required and a certain risk of conflict thus exists in dimensioning and protecting generators and transformers. The step-up transformer has no air gap and is therefore sensitive to saturation as a result of high voltage or geomagnetic currents. The transformer also consumes part of the reactive power of the generator, both at normal and abnormal operation. The majority of the active losses appear in the conductors of the armature circuit and the step-up transformer, while the core losses are relatively small in both devices. One complication here is that the losses are normally developed at medium and high voltage and are therefore more difficult to cool away than if they had been developed at earth potential.
The object of the present invention is to achieve a synchronous machine for power and/or voltage control and a method for power and/or voltage control in order to avoid voltage collapse in power systems.
According to the invention, thus, the synchronous machine is designed so that the thermally based rotor current limit is raised with respect to the thermally based stator current limit such that either the intersection with the thermally based stator current limit in the capability graph is at a power factor value considerably below power factor value, or the rotor current limit is raised above the stator current limit such that the two limits do not intersect. If the rotor and stator current limits intersect at the power factor zero in the capability graph as shown in FIG. 2, or if the rotor current limit is raised above the stator current limit, the stator current limit will be limiting for all overexcited operation.
In the following xe2x80x9ccablexe2x80x9d shall refer to high-voltage, insulated electric conductors comprising a core having a number of strand parts of conducting material such as copper, for instance, an inner semiconducting layer surrounding the core, a high-voltage insulating layer surrounding the inner semiconducting layer, and an outer semi-conducting layer surrounding the insulating layer. A synchronous machine with a stator winding which comprises this type of cable can be designed for direct connection to the power network at higher voltages than with conventional machines, thus eliminating the need for a step-up transformer. In the case of reactive power production it is advantageous to use a machine designed for direct connection to transmission level, since the reactive power consumed in the step-up transformer in the conventional plant instead can be delivered to the power network with a machine according to the invention.
The advantages of the invention are particularly noticeable in a machine wound with a cable of the type described above, particularly a cable having a diameter within the interval 20-200 mm and a conducting area within the interval 80-3000 mm2. Such applications of the invention thus constitute preferred embodiments thereof.
Raising the rotor current limit has a number of advantages for a synchronous machine. It enables direct measurement of limiting stator temperatures, for instance. This is considerably more difficult if the limiting temperatures are located in the rotor since it is difficult to measure, or in any other way communicate with a rotating object. Furthermore, reducing active power enables more reactive power to be produced. This is also possible with conventional rotor dimensioning but more MVAr per reduced MW results in this case, as can be seen in the curves in FIGS. 1 and 2.
A number of other advantages are also gained by raising the rotor current limit, specific to this type of machine. The time constants for heating (and cooling) the stator are large in comparison with a conventional machine. This means that the machine, with conventional stator current limiters, can be run overloaded or longer than a conventional machine without damaging temperatures being reached. Simulations indicate that the stator safely can be overloaded 80% for 15 minutes in some cases. This extended time period can be utilized to take action either to reduce the system""s need for reactive power, or to increase the production of reactive power. It is also easier to implement forced cooling of the stator of the machine. A machine of this type has a degree of efficiency comparable with that of a conventional machine, i.e. the stator losses are approximately equivalent. While a conventional machine has primarily conductor losses, this type of machine has less conductor losses and more core losses. Since the core losses are developed at earth potential they are easier to cool away. A cooling machine can be used, for instance, for forced cooling in situations with high core temperatures.
With conventional current limiters the time period contributed by the time constant for heating, can be utilized to reduce the active power and thus enable increased and/or prolonged production of reactive power. The need for reducing the field is thus less and, in the best case, is eliminated.
With direct temperature measurement or temperature estimation (or a combination thereof) we can pass from using the term xe2x80x9cstator current limitxe2x80x9d to talking about stator temperature limit(s). Since it is the stator temperature (in critical points), and not the stator current, that is limiting, this offers a number of advantages. The general tendency to set the limiter conservatively can thus be lessened since it is the primary quantity that is known and not a derivative. With a conventional current limiter no consideration can be taken to the temperature of the machine when the current limit is exceeded, i.e. no consideration can be taken to the fact, for instance, that the machine was started shortly before the current limit was exceeded, or that the load was low shortly before. This can be avoided by using stator temperature limit(s) instead. Cooling of the machine is dimensioned so that the stator in continuous rated operation does not exceed a certain temperaturexe2x80x94let us call this the rated temperature. This temperature is consciously set conservatively, i.e. the stator (insulation) can withstand higher temperatures for long periods of time. If the temperature in the critical points is known the machine can be run above rated operation for relatively long periods.
Dimensioning the rotor with salient poles (hydroelectric generators) in synchronous machines according to the invention is facilitated by the fact that the inner diameter of the stator can be made larger than in conventional machines since the stator winding is composed of cable in which the insulation takes up more space. It is thus possible to design the stator for this type of synchronous machine in accordance with conventional dimensioning procedures and change only the design of the rotor so that the rotor current limit is raised in the desired manner.
For a synchronous machine incorporating an air-cooled rotor with salient poles, this can be done, for instance, by utilizing the extra space to wind extra turns of the field winding in order to increase the magnetic pole voltage. A certain number of turns in the field winding then consist of cooling turns, thus increasing the cooled surface of the field winding. If the extra turns are provided with the same proportion of cooling turns, as the other turns the temperature increase in the field winding can be kept at the same level as in a conventional dimensioning procedure, despite the magnetic pole voltage being raised.
For a synchronous machine with cylindrical rotor (turbo-rotor) the rotor current limit can be increased by making the machine longer, for instance.